1. Field
The present application generally relates to examining a structure formed on a semiconductor wafer using optical metrology, and, more particularly, to determining one or more features of an in-die structure formed on a semiconductor wafer using optical metrology.
2. Description of the Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine a feature of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a test structure, such as a grating array, in a test pad, in proximity to a die on a semiconductor wafer, an optical metrology system is used to determine the profile of the test structure. By determining the profile of the test structure, the quality of the fabrication process utilized to form the test structure, and by extension the die proximate the test structure, can be evaluated.
In conventional optical metrology, it is typically assumed that the test structure in the test pad has the same profile as a structure in the die (i.e., an in-die structure). This assumption has limitations because the local environment can affect the fabrication process and alter the profile of the test structure in the test pad relative to the in-die structure.